Role of Snf1p in regulation of intracellular sorting of the lactose and galactose transporter Lac12p in Kluyveromyces lactis

The binary response of the GAL/MEL genetic switch of Saccharomyces cerevisiae is critically dependent on Gal80p-Gal4p interaction

Studies on the Saccharomyces cerevisiae GAL/MEL genetic switch have revealed that its bistability is dependent on ultrasensitivity that can be altered or abolished by disabling different combinations of nested feedback loops. In contrast, we have previously demonstrated that weakening of the interaction between Gal80p and Gal4p alone is sufficient to abolish the ultrasensitivity (Das Adhikari et al. 2014). Here, we demonstrate that altering the epistatic interaction between Gal80p and Gal4p also abolishes the bistability, and the switch response to galactose becomes graded instead of binary. However, the GAL/MEL switch of wild-type and epistatically altered strains responded in a graded fashion to melibiose. The properties of the epistatically altered strain resemble Kluyveromyces lactis, which separated from the Saccharomyces lineage 100 mya before whole-genome duplication (WGD). Based on the results reported here, we propose that epistatic interactions played a crucial role in the evolution of the fine regulation of S. cerevisiae GAL/MEL switch following WGD.

Signaling pathways and posttranslational modifications of tau in Alzheimer's disease: the humanization of yeast cells

In the past decade, yeast have been frequently employed to study the molecular mechanisms of human neurodegenerative diseases, generally by means of heterologous expression of genes encoding the relevant hallmark proteins. However, it has become evident that substantial posttranslational modifications of many of these proteins are required for the development and progression of potentially disease relevant changes. This is exemplified by the neuronal tau proteins, which are critically involved in a class of neuro-degenerative diseases collectively called tauopathies and which includes Alz-heimer's disease (AD) as its most common representative. In the course of the disease, tau changes its phosphorylation state and becomes hyperphosphory-lated, gets truncated by proteolytic cleavage, is subject to O-glycosylation, sumoylation, ubiquitinylation, acetylation and some other modifications. This poses the important question, which of these posttranslational modifications are naturally occurring in the yeast model or can be reconstituted by heterol-ogous gene expression. Here, we present an overview on common modifica-tions as they occur in tau during AD, summarize their potential relevance with respect to disease mechanisms and refer to the native yeast enzyme orthologs capable to perform these modifications. We will also discuss potential approaches to humanize yeast in order to create modification patterns resembling the situation in mammalian cells, which could enhance the value of Saccharomyces cerevisiae and Kluyveromyces lactis as disease models.

Signaling pathways and posttranslational modifications of tau in Alzheimer's disease: the humanization of yeast cells

In the past decade, yeast have been frequently employed to study the molecular mechanisms of human neurodegenerative diseases, generally by means of heterologous expression of genes encoding the relevant hallmark proteins. However, it has become evident that substantial posttranslational modifications of many of these proteins are required for the development and progression of potentially disease relevant changes. This is exemplified by the neuronal tau proteins, which are critically involved in a class of neuro-degenerative diseases collectively called tauopathies and which includes Alz-heimer's disease (AD) as its most common representative. In the course of the disease, tau changes its phosphorylation state and becomes hyperphosphory-lated, gets truncated by proteolytic cleavage, is subject to O-glycosylation, sumoylation, ubiquitinylation, acetylation and some other modifications. This poses the important question, which of these posttranslational modifications are naturally occurring in the yeast model or can be reconstituted by heterol-ogous gene expression. Here, we present an overview on common modifica-tions as they occur in tau during AD, summarize their potential relevance with respect to disease mechanisms and refer to the native yeast enzyme orthologs capable to perform these modifications. We will also discuss potential approaches to humanize yeast in order to create modification patterns resembling the situation in mammalian cells, which could enhance the value of Saccharomyces cerevisiae and Kluyveromyces lactis as disease models.

Yeast on the milky way: genetics, physiology and biotechnology of Kluyveromyces lactis

The milk yeast Kluyveromyces lactis has a life cycle similar to that of Saccharomyces cerevisiae and can be employed as a model eukaryote using classical genetics, such as the combination of desired traits, by crossing and tetrad analysis. Likewise, a growing set of vectors, marker cassettes and tags for fluorescence microscopy are available for manipulation by genetic engineering and investigating its basic cell biology. We here summarize these applications, as well as the current knowledge regarding its central metabolism, glucose and extracellular stress signalling pathways. A short overview on the biotechnological potential of K. lactis concludes this review.

Regulations of sugar transporters: insights from yeast

Transport across the plasma membrane is the first step at which nutrient supply is tightly regulated in response to intracellular needs and often also rapidly changing external environment. In this review, I describe primarily our current understanding of multiple interconnected glucose-sensing systems and signal-transduction pathways that ensure fast and optimum expression of genes encoding hexose transporters in three yeast species, Saccharomyces cerevisiae, Kluyveromyces lactis and Candida albicans. In addition, an overview of GAL- and MAL-specific regulatory networks, controlling galactose and maltose utilization, is provided. Finally, pathways generating signals inducing posttranslational degradation of sugar transporters will be highlighted.

Ght2⁺ is required for UDP-galactose synthesis from extracellular galactose by Schizosaccharomyces pombe

Schizosaccharomyces pombe has eight hexose transporter genes, ght1 (+) to ght8 (+). Here we report that ght2 (+), which is highly expressed in the presence of glucose, is essential for UDP-galactose synthesis from extracellular galactose when cells grow on glucose. The galactosylation defect of a uge1Δ mutant defective in synthesis of UDP-galactose from glucose was suppressed in galactose-containing medium, but disruption of ght2 (+) in the uge1Δ mutant reversed suppression of the galactosylation defect. Expression of Saccharomyces cerevisiae GAL2 in uge1Δght2Δ cells suppressed the defective galactosylation phenotype in galactose-containing medium. These results indicate that galactose is transported from the medium to the cytosol in a Ght2-dependent manner, and is then converted into UDP-galactose.

SNF1/AMPK pathways in yeast

The SNF1/AMPK family of protein kinases is highly conserved in eukaryotes and is required for energy homeostasis in mammals, plants, and fungi. SNF1 protein kinase was initially identified by genetic analysis in the budding yeast Saccharomyces cerevisiae. SNF1 is required primarily for the adaptation of yeast cells to glucose limitation and for growth on carbon sources that are less preferred than glucose, but is also involved in responses to other environmental stresses. SNF1 regulates transcription of a large set of genes, modifies the activity of metabolic enzymes, and controls various nutrient-responsive cellular developmental processes. Like AMPK, SNF1 protein kinase is heterotrimeric. It is phosphorylated and activated by the upstream kinases Sak1, Tos3, and Elm1 and is inactivated by the Reg1-Glc7 protein phosphatase 1. Further regulation of SNF1 is achieved through autoinhibition and through control of its subcellular localization. Here we review the current understanding of SNF1 protein kinase pathways in Saccharomyces cerevisiae and other yeasts.

Transcriptomic analysis of extensive changes in metabolic regulation in Kluyveromyces lactis strains

Genome-wide analysis of transcriptional regulation is generally carried out on well-characterized reference laboratory strains; hence, the characteristics of industrial isolates are therefore overlooked. In a previous study on the major cheese yeast Kluyveromyces lactis, we have shown that the reference strain and an industrial strain used in cheese making display a differential gene expression when grown on a single carbon source. Here, we have used more controlled conditions, i.e., growth in a fermentor with pH and oxygen maintained constant, to study how these two isolates grown in glucose reacted to an addition of lactose. The observed differences between sugar consumption and the production of various metabolites, ethanol, acetate, and glycerol, correlated with the response were monitored by the analysis of the expression of 482 genes. Extensive differences in gene expression between the strains were revealed in sugar transport, glucose repression, ethanol metabolism, and amino acid import. These differences were partly due to repression by glucose and another, yet-unknown regulation mechanism. Our results bring to light a new type of K. lactis strain with respect to hexose transport gene content and repression by glucose. We found that a combination of point mutations and variation in gene regulation generates a biodiversity within the K. lactis species that was not anticipated. In contrast to S. cerevisiae, in which there is a massive increase in the number of sugar transporter and fermentation genes, in K. lactis, interstrain diversity in adaptation to a changing environment is based on small changes at the level of key genes and cell growth control.